This invention relates to a novel method and an apparatus for operating a circulating fluidized bed system.
Circulating fluidized bed (CFB) systems such as CFB combustors include a combustion chamber having a fast fluidized bed of solid particles therein. A particle separator is connected to a discharge opening in the upper part of the combustion cheer, for separating solid particles from the suspension of flue gases and entrained solid material being discharged from the combustion chamber. A return duct is connected between the particle separator and the lower part of the combustion chamber for recirculating separated solid particles from the particle separator into the combustion chamber. A gas outlet is arranged in the particle separator for discharging flue gases.
Cyclone separators are commonly used as particle separators. A dip leg recirculates the separated particles from the cyclone to the lower part of the combustion chamber. A loop seal has to be arranged in the dip leg in order to prevent gases from flowing from the combustion chamber backward into the cyclone therethrough. The loop seal constructions are very large and complicated. It has also been suggested to use L-valves as loop seals. The L-valve is, however, also space consuming, as a rather long connection channel filled with bed material is needed between the return duct and the combustion chamber in order establish a loop seal.
The circulating fluidized bed reactors are used in a variety of different combustion, heat transfer, chemical or metallurgical processes. Depending on the process, different bed materials are fluidized or circulated in the system. In combustion processes particulate fuel such as coal, coke, lignite, wood, waste or peat, as well as other particulate matter such as sand, ash, sulfur absorbent, catalyst or metal oxides can be the constituents of the fluidized bed. The velocity in the combustion chamber usually is in the range of 3,5 to 10 m/s, but can be substantially higher.
Typically heat is recovered from fluidized bed combustion processes by heat transfer surfaces in the combustion chamber and in the convection section arranged in the gas pass after the particle separator. In circulating fluidized bed (CFB) combustors or boilers the peripheral walls of the combustion chamber are usually made as membrane walls in which vertical tubes are combined by flat plate material or fins to form evaporating surfaces. Additional heat transfer surfaces such as superheaters may be arranged within the upper part of the combustion chamber for superheating the steam.
Additional superheaters as well as reheaters, preheaters and air preheaters are arranged in the convection section. It has also been suggested to forth the return duct of heat transfer surfaces.
The heat transfer surfaces are normally designed to give optimal superheated steam also at a low load range. At higher loads steam production is then controlled by spraying water in the convection section.
Superheating at low load often constitutes a problem. The combustion chamber exit gas temperature decreases with decreasing load and the superheaters in the convection section do not have enough capacity to provide the desired results. Additional superheaters arranged in the combustion chamber would increase costs and control problems in the boiler improperly. Additional heat transfer surfaces within the combustion chamber would further decrease the temperature of the flue gases, which still contain unburned fuel, to e.g. 700.degree. to 750.degree. C., which would have an negative effect on NOX and N.sub.2 O reduction.
Additional separate heat transfer surfaces within a fluidized bed would on the other hand also be exposed to the high velocity (3-10 m/s or even higher) flow of gas and particles therein. Corrosion and erosion would cause sever problems. Any heat transfer surface arranged within the combustion chamber would have to be made of heat resistant material, most probably also protected by some erosion resistant material. Such heat transfer surfaces would thus become very heavy and expensive. The corrosion constitutes a severe problem in the gas atmosphere in the combustion chamber, when burning fuels containing gaseous chlorine and alkali components.
Especially in pressurized applications it is even less desirable to have to add heat transfer surfaces and increase the size of the combustor, which leads to a need to increase the size of the pressure vessel, as well. In pressurized applications, having smaller combustion chambers, heat transfer surfaces are already very close to each other. It would therefor be very difficult to add any additional heat transfer surface into the combustion chamber. A very compact arrangement of heat transfer surfaces also prevents horizontal mixing of bed material within the combustion chamber and results in decreased combustion efficiency. Besides space problems, clogging may also become a problem if heat transfer surfaces are arranged very close to each other.
It has been suggested to use external heat exchangers (EHE) for increasing the superheating capacity. In such superheaters in a separate fluidized bed of hot circulating solid material, the solid material is introduced into the EHE from the particle separator. The suggested external heat exchangers would be large and expensive as well as difficult to control independently from the main combustion process. Erosion would also constitute a problem when exposing heat transfer surfaces to a fluidized bed of large hot particles. Further at very low loads the amount of solid material being discharged with the flue gases from the combustion chamber and introduced in the EHE would decrease to such a level that superheating could be achieved. A simpler and less expensive solution is needed.
It is an object of the present invention to provide a method and an apparatus for operating circulating fluidized bed systems in which the above mentioned drawbacks are minimized.
It is especially an object of the present invention to provide an improved loop seal arrangement for circulating fluidized bed systems.
It is further an object of the present invention to provide an improved method for heat recovery in circulating fluidized bed systems.
It is still further an object of the present invention to provide an improved method for controlling the heat recovery in a circulating fluidized bed system.
It is thereby also an object of the present invention to provide an improved method for superheating of steam in a circulating fluidized bed boiler system, at different load conditions.